Thin film resistors are commonly used in integrated circuits. Typically, a plurality of thin film resistors are formed on an electrically insulating substrate layer, which typically is an oxide layer formed on the integrated circuit chip. The thin film resistors, in general, are formed in a specific area on the substrate layer to, in general, extend parallel to each other. It is desirable that the thin film resistors be located relatively close to each other for two important reasons, firstly, to minimise the area occupied by the thin film resistors on the integrated circuit chip, in order to minimise the overall die area required, and secondly, to minimise the effect of process variations on the thin film resistors, which can result in mismatch and other related problems.
However, even locating the thin film resistors close to each other does not completely avoid the effects of process variation, which can result in mismatch of the resistors on the same chip, and from chip to chip. Thus, trimming of the resistance of thin film resistors must be carried out after the film resistors have been formed on the integrated circuit chip. This, thus, requires that provision must be made during the formation of the thin film resistors for facilitating subsequent trimming of the resistance of the film resistors.
One method for forming thin film resistors which lends itself to subsequent trimming requires that the thin film resistors be formed with a sidewardly projecting tab which is subsequently trimmed for increasing the resistance of the resistors. Such prior art thin film resistors are illustrated in FIG. 1 and are indicated by the reference numeral 100. Each thin film resistor 100 is formed on an electrically insulating substrate 101, which typically is of an oxide material, such as silicon dioxide. The thin film resistors may be of any suitable material, for example, silicon chrome, which may be doped or otherwise. The thin film resistors are typically formed by physical vapour deposition (PVD), by chemical vapour deposition (CVD) or by sputtering, and are deposited to a depth, which is maintained constant and depends on the process. The length and width of the thin film resistors are dictated by the desired resistance values of the resistors. Typically, such thin film resistors are deposited to a depth of up to 100 Angstroms. The thin film resistors 100 extend between respective pairs of electrical contact pads 102 and 103, and each thin film resistor 100 is provided with a sidewardly extending tab 104 for facilitating trimming of the resistance of the thin film resistor 100. In order to minimise the spacing between the thin film resistors 100, the thin film resistors 100 are arranged in pairs with the tabs 104 of adjacent pairs facing each other and being staggered along the respective lengths of the thin film resistors 100.
The effect of the tabs 104 on the thin film resistors 100 is to reduce the current density of current flowing through the thin film resistors 100 adjacent the area of the tabs 104, and thus trimming of the tabs 104 provides relatively high resolution trimming.
Trimming of each thin film resistor 100 is generally carried out by progressively extending a trimming slot 105 into the tab 104 of the thin film resistor 100 being trimmed. The trimming slots 105 are formed by a laser light beam, and in general are formed to extend parallel to the thin film resistors 100. By virtue of the fact that the current density is reduced in the thin film resistors 100 adjacent the area of the tabs 104, a relatively wide resistance value trim range is achievable, as well as relatively high trim resolution.
While alignment techniques for aligning a laser light beam with a tab 104 to be trimmed have been improved over the years, the size of the active high energy spot of the laser light beam which actually forms the trimming slot 105 is still relatively large, and accordingly, it is essential that the spacing between the tabs 104 of adjacent thin film resistors 100 must be sufficient to avoid any danger of the laser light beam as it is forming a trimming slot 105 in the tab 104 of one of the thin film resistors 100 damaging the adjacent thin film resistor 100. Typically, the high energy laser spot is of diameter of the order of three microns to five microns. Thus, while the spacing between the thin film resistors can be reduced somewhat by arranging the thin film resistors 100 with the tabs 104 of adjacent pairs of resistors facing each other, nonetheless, the thin film resistors 100 must be spaced apart a sufficient distance to avoid unintentional trimming of a thin film resistor 100 adjacent another thin film resistor 100 the tab 104 of which is being trimmed.
Typically, the transverse distance A between the tab 104 of one thin film resistor 100 and the adjacent thin film resistor 100 should be at least nine microns, while the longitudinal distance B between the tabs 104 of adjacent thin film resistors 100 should be of the order of ten microns. Additionally, the tab 104 of each thin film resistor 100 should be a longitudinal distance C from the closest electrical contact pad 102 or 103 of at least nine microns.
Accordingly, while this prior art method of forming and trimming thin film resistors provides a relatively wide trim range as well as relatively high trim resolution, nonetheless, it still requires a relatively large spacing between the thin film resistors, which in turn results in a relatively large die area to accommodate the thin film resistors, and potential mismatch between the thin film resistors.
There is therefore a need for a film resistor which addresses this problem.
The present invention is directed towards providing a film resistor which can be located relatively close to an adjacent film resistor, and which can be subsequently trimmed. The invention is also directed towards a method for forming and trimming such a film resistor, and the invention is also directed towards providing an integrated circuit comprising a plurality of film resistors formed thereon.